One of the largest environmental concerns associated with raising poultry for production in confined enclosures is the accumulation of ammonia gas (NH3). Volatilization of NH3 inside poultry housing often results in an excessive accumulation of NH3 in the air, which can negatively affect the health of both workers and birds (Kirychuk et al., Journal of Occupational and Environmental Medicine 48 (7):741-748, 2006; Ritz et al., Journal of Applied Poultry Research 13:684-692, 2004; Rylander and Carvalheiro, International Archives of Occupational and Environmental Health 79 (6):487-490) 2006).
Numerous studies have shown the detrimental effect of high levels of NH3 on bird productivity (Dawkins et al., Nature 427 (6972):342-344, 2004; Ritz et al., supra; Wathes et al., Transactions of the American Society of Agricultural Engineers 45 (5):1605-1610, 2002; Yahav, Animal Research 53:289-293, 2004). Although increased ventilation can lower the NH3 in poultry houses to safe levels, it is expensive due to energy costs during winter months (Moore et al., 1995, Journal of Environmental Quality, Volume 24, 293-300). Since NH3 cannot be effectively contained within the house structure, NH3 emissions may contribute to air pollution, atmospheric deposition, and health concerns for near-by residents (Nahm, 2003, World's Poultry Science Journal, Volume 59, 77-88; Wheeler et al., 2006, Transactions of the American Society of Agricultural Engineers, Volume 49(5), 1495-1512; Williams et al., 1999, Reviews of Environmental Contamination and Toxicology, Volume 162, 105-157; Wing and Wolf, 2000, Environmental Health Perspectives, Volume 108(3), 233-238).
Ammonia levels as low as 20 ppm have been found to compromise the immune and respiratory systems of chickens, making them more susceptible to disease. High levels of ammonia also negatively affect their feed conversion and weight gain. As a result of all these negative impacts on performance, recommended ammonia concentrations in poultry barns should be well below 25 ppm.
High levels of ammonia may also pose a risk to the health of agricultural workers in chicken rearing facilities; exposure to ammonia can irritate the respiratory tract and eyes, even at low levels. Therefore, the Federal Occupational Safety and Health Administration (OSHA) permissible worker exposure limit for ammonia is 50 ppm over an 8-hour period and the American Conference of Governmental Industrial Hygienists (ACGIH) has established a short-term (15-min) exposure limit of 35 ppm.
Current NH3 abatement technologies used in livestock houses rely on the ventilation systems and treatment of the exhaust air after leaving the house to remove nitrogen. Typically such systems are large requiring a lot of power that allows for an exchange range from 275 to 451 cubic feet of air per second depending on ambient temperatures (colder and warmer, respectively), assuming an average weight per broiler of 1.3 kg and 20000 broilers per house (American Society of Agricultural and Biological Engineers, Design of Ventilation Systems for Poultry and Livestock Shelters, ASABE Standard Practices, ASAE EP270.5 December 1986 (R2008), 1986). Such massive ventilation allows for the dilution of indoor ammonia levels and it does increase the removal amounts of ammonia from poultry facilities. However, ammonia releases from such facilities in this manner to the atmosphere is expensive in the colder months and throughout the year can cause environmental problems, such as acid precipitation, fine particulate matter formation (particulate matter with an aerodynamic diameter less than ten microns in size), and nitrogen deposition into aquatic systems. The accumulated effects of ventilation contribute to a reduction in the quality of life and raise health concerns for near-by residents.
A second strategy includes treating the NH3 in the exhaust air from enclosures using scrubbing or filtration techniques, thus preventing NH3 release into the environment. This technique consists of forcing the ventilated air through an NH3 trap, such as an acidic solution (scrubbers), or through a porous filter with nitrifying biofilms that oxidize NH3 to nitrate (biotrickling or organic filters) (Chen et al., Chemosphere 58 (8):1023-1030, 2005; Melse and Ogink, Transactions of the ASAE 48 (6):2303-2313, 2005; Ndegwa et al., Biosystems Engineering 100:453-469, 2008; Pagans et al., Chemical Engineering Journal 113 (2-3):105-110, 2005). The process is costly in winter months when it is necessary to heat enclosures to maintain production. In addition, recent research has shown that NH3 concentrations close to the litter surface (<20 cm), where the birds are exposed, can be up to one order of magnitude higher than in the bulk house air.
The third technology is to selectively pull and treat the air near the litter surface, where NH3 levels are more concentrated, using dedicated ventilation systems independent of the house ventilation system (Lahav et al., Water Air Soil Pollution, Volume 191, 183-197, 2008). A significant departure from the methods described above is the concept of removing NH3 using manifolds that extract only the air close to the litter independent of the house ventilation system. These systems require redundancy, additional positive air extraction equipment and are thus not cost effective.
A fourth form of abatement is to add chemical amendments directly to the poultry litter to prevent NH3 volatization, without the need of additional ventilation to move NH3. These amendments act by either inhibiting microbial transformation of urea or uric acid into NH3 or by acidifying and neutralizing it. Several chemical amendments have been widely used for their ability to control or reduce NH3 release from poultry litter and manure, such as AL2(SO4)3.14H20 (Al+Clear®), NaHSO4 (PLT®), and acidified clays (Poultry Guard®) (Cook et al., Journal of Environmental Quality 37:2360-2367, 2008; Moore et al., 1995, supra; Moore et al., Journal of Environmental Quality 29:37-49, 2000; Shah et al., Poultry litter amendments, edited by N. C. C. E. Service. Raleigh, N.C.: North Carolina State University, 2006). Although N is conserved unvolatilized in the poultry litter, NH3 is not recovered as a separate product as with the scrubbing techniques. Recovery of NH3 is a desirable feature because it can be exported off the farm, solving problems of N surpluses in concentrated poultry production regions.
Conservation and recovery of nitrogen (N) is also important in agriculture because of the high cost of producing and acquiring commercial NH3 fertilizers. Thus, there is a desire to improve technologies for abating NH3 emissions from confined poultry operations by capturing and recovering nitrogen.
While, various systems have been developed for removing NH3 from animal litter, there still remains a need in the art for different abatement systems that removes NH3 from gaseous nitrogen producing systems and recovers the N in a concentrated purified form, but is not dependent on intense air movement. The present invention, different from prior art systems, provides such systems using hydrophobic gas-permeable membranes and circulated acidic solutions to produce concentrated ammonium salt.